Hypoplastic left heart syndrome (HLHS) is a problem with the heart’s structure that is present at birth (congenital). It is a group of related defects that, together, mean that the left side of the heart is underdeveloped. Normally, oxygen-poor blood is pumped through the right side of the heart to the lungs, where it gains oxygen and returns to the left side of the heart. The oxygen-rich blood is then pumped from the left side of the heart to the rest of the body. At birth, all babies also have two connections, or shunts, between the two sides of the heart; however, within a few days of birth these connections close.